Esters of polyhydric alcohols, also known as polyol esters, find a variety of uses on a large scale in industry, for example as plasticizers or lubricants. The selection of suitable starting materials allows the physical properties, for example boiling point or viscosity, to be controlled, and the chemical properties, such as hydrolysis resistance or stability to oxidative degradation, to be taken into account. Polyol esters can also be tailored to the solution of specific performance problems. Detailed overviews of the use of polyol esters can be found, for example, in Ullmann's Encyclopaedia of Industrial Chemistry, 5th edition, 1985, VCH Verlagsgesellschaft, Vol. A1, pages 305-319; 1990, Vol. A15, pages 438-440, or in Kirk 3rd Othmer, Encyclopaedia of Chemical Technology, edition, John Wiley & Sons, 1978, Vol. 1, pages 778-787; 1981, Vol. 14, pages 496-498.
The use of polyol esters as lubricants is of great industrial significance, and they are used particularly for those fields of use in which mineral oil-based lubricants meet the requirements made only incompletely. Polyol esters are used especially as turbine engine and instrument oils. Polyol esters for lubricant applications are based frequently on 1,3-propanediol, 1,3-butanediol, 1,4-butanediol, 1,2-hexanediol, 1,6-hexanediol, neopentyl glycol, trimethylolpropane, pentaerythritol, 2,2,4-trimethyl-pentane-1,3-diol, glycerol or 3(4),8(9)-dihydroxy-methyltricyclo[5.2.1.02,6]decane, also known as TCD alcohol DM, as the alcohol component.
Polyol esters are also used to a considerable degree as plasticizers. Plasticizers find a variety of uses in plastics, coating materials, sealing materials and rubber articles. They interact physically with high molecular weight thermoplastic substances, without reacting chemically, preferably by virtue of their swelling and dissolution capacity. This forms a homogeneous system, the thermoplastic range of which is shifted to lower temperatures compared to the original polymers, one result being that the mechanical properties thereof are optimized, for example deformation capacity, elasticity and strength are increased, and hardness is reduced.
In order to open up the widest possible fields of use to plasticizers, they must fulfil a series of criteria. They should ideally be odorless, colorless, and light-, cold- and heat-resistant. Moreover, it is expected that they are insensitive to water, comparatively nonflammable and not very volatile, and are not harmful to health. Furthermore, the production of the plasticizers should be simple and, in order to meet ecological requirements, avoid waste substances, such as by-products which cannot be utilized further and wastewaters comprising pollutants.
A specific class of polyol esters (they are referred to as G esters for short) contains diols or ether diols as the alcohol component, for example ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, 1,2-propylene glycol or higher propylene glycols. They can be prepared in different ways. In addition to the reaction of alcohol and acid, optionally in the presence of acidic catalysts, further processes are employed in practice to obtain G esters, including the reaction of diol with acid halide, the transesterification of a carboxylic ester with a diol, and the addition of ethylene oxide onto carboxylic acids (ethoxylation). In industrial manufacture, only the direct reaction of diol and carboxylic acid and the ethoxylation of carboxylic acids have become established as production processes, preference usually being given to the esterification of diol and acid. This is because this process can be performed with no particular complexity in conventional chemical apparatus, and it affords chemically homogeneous products. Compared to this, ethoxylation requires extensive and costly technical equipment. Ethylene oxide is a very reactive chemical substance. It can polymerize explosively and forms explosive mixtures with air within very wide mixing ranges. Ethylene oxide irritates the eyes and respiratory pathways, leads to chemical burns and to liver and kidney damage, and is carcinogenic. The handling thereof therefore entails extensive safety measures. Moreover, scrupulous cleanliness of storage apparatus and reaction apparatus has to be ensured, in order to rule out the formation of undesired impurities as a result of side reactions of the ethylene oxide with extraneous substances. Finally, the reaction with ethylene oxide is not very selective, since it leads to mixtures of compounds of different chain length.
The direct esterification of alcohols with carboxylic acids is one of the basic operations in organic chemistry. In order to increase the reaction rate, the conversion is typically performed in the presence of catalysts. The use of one of the reactants in excess and/or the removal of the water formed in the course of the reaction ensures that the equilibrium is shifted in accordance with the law of mass action to the side of the reaction product, i.e. of the ester, which means that high yields are achieved.
Comprehensive information regarding the preparation of esters of polyhydric alcohols, also including esters of ethylene glycols and fatty acids, and regarding the properties of selected representatives of these compound classes can be found in Goldsmith, Polyhydric Alcohol Esters of Fatty Acids, Chem. Rev. 33, 257 ff. (1943). For example, esters of diethylene glycol, of triethylene glycol and of polyethylene glycols are prepared at temperatures of 130 to 230° C. over reaction times of 2.5 to 8 hours. To remove the water of reaction, carbon dioxide is used. Suitable catalysts mentioned for the esterification of polyhydric alcohols are inorganic acids, acidic salts, organic sulphonic acids, acetyl chloride, metals or amphoteric metal oxides. The water of reaction is removed with the aid of an entraining agent, for example toluene or xylene, or by introducing inert gases such as carbon dioxide or nitrogen.
The production and the properties of fatty acid esters of the polyethylene glycols are discussed by Johnson (edit.), Fatty Acids in Industry (1989) Chapter 9, Polyoxyethylene Esters of Fatty Acids, and a series of preparative details are given. Higher diester concentrations are achieved by the increase in the molar ratio of carboxylic acid to glycol. Suitable measures for removing the water of reaction are azeotropic distillation in the presence of a water-immiscible solvent, heating while passing through an inert gas, or performing the reaction under reduced pressure in the presence of a desiccant. When the addition of catalysts is dispensed with, longer reaction times and higher reaction temperatures are required. Both reaction conditions can be made milder by the use of catalysts. In addition to sulphuric acid, organic acids such as p-toluenesulphonic acid and cation exchangers of the polystyrene type are the preferred catalysts. The use of metal powders, such as tin or iron, is also described. According to the teaching from U.S. Pat. No. 2,628,249, color problems in the case of catalysis with sulphuric acid or sulphonic acids can be alleviated when working in the presence of activated carbon.
One procedure in which esters of diethylene glycol and of triethylene glycol and of caprylic acid are prepared without addition of catalyst is known from U.S. Pat. No. 2,469,446. The esterification temperature is in the range from 270 to 275° C. and the water of reaction is driven out by means of a carbon dioxide stream.
In the reaction regime in which the addition of a catalyst is dispensed with, a molar excess of the particular carboxylic acid is generally employed, which, owing to its acidity, also acts as a catalyst.
For the removal of the water of reaction formed in the formation of ester from the polyol and the carboxylic acids, various processes are known. For example, the water of reaction formed is distilled out of the reaction vessel together with the excess carboxylic acid and passed into a downstream phase separator in which carboxylic acid and water separate according to their solubility properties. In some cases, the carboxylic acid used also forms an azeotrope with water under the reaction conditions, and is capable of removing the water of reaction as an entraining agent. Other methods employed include azeotropic distillation in the presence of an added water-immiscible solvent, heating of the reaction mixture while passing through an inert gas, the reaction of the polyol and carboxylic acid starting materials under reduced pressure or in the presence of a desiccant. Especially the removal of water by azeotropic distillation has been found to be useful for the establishment of the equilibrium in the preparation of polyol esters. According to the procedure known from DE 199 40 991 A1, the water-immiscible solvent which acts as an entraining agent and must have a boiling point of less than 112° C. is added to the reaction mixture only on attainment of a temperature of at least 140° C.
In the industrial process, the mixture of water and carboxylic acid removed is separated in a phase separator into the organic and aqueous phases, the aqueous phase is discharged and the carboxylic acid is recycled back into the esterification reaction. For the workup of the crude ester, for example, U.S. Pat. No. 5,324,853 A1 proposes removing excess carboxylic acid by means of passage of nitrogen or steam, adding an adsorbent, neutralizing residual organic acid with a base, and filtering off solids obtained. The residual amounts of acid present in the filtrate are removed with the passage of steam or nitrogen with simultaneous application of a reduced pressure and recycled back into the esterification reaction. Solids obtained in the vacuum treatment are removed in a final fine filtration. One task of the adsorbent added, for example activated carbon, is to improve the color of the polyol ester.
According to the procedure known from U.S. Pat. No. 2,469,446 A1, the crude ester obtained after removal of the water of reaction and of excess, unconverted starting materials, for example carboxylic acid, is first treated with an alkaline reagent, for example with an aqueous sodium carbonate or sodium hydroxide solution, in order to remove last residues of acidic constituents. After washing with water, and treatment with bleaching earth and activated carbon, the last traces of odorous substances can be removed by applying reduced pressure at elevated temperature. In some cases, the treatment with bleaching agents and activated carbon has to be repeated more than once in order to produce polyol esters with satisfactory color properties.
Measures for improving the color of crude esters, such as oxidation, for example with hydrogen peroxide or ozone, and the adsorption of activated carbon, are known from the general prior art, for example from H. Suter, Phthalsäureanhydrid and seine Verwendung [Phthalic anhydride and use thereof], Dr. Dietrich Steinkopf Verlag, Darmstadt 1972. To improve the color of ester compounds based on polyols, WO 94/18153 A1 proposes a subsequent treatment with an aqueous hydrogen peroxide solution. The action of ozone to lighten the color is discussed, for example, in DE 27 29 627 A1.
Owing to the quality criteria described at the outset for polyol esters, the process steps in the esterification stage with removal of the water of reaction and in the workup of the crude ester are very important process features, since the adjustment of these process steps influences the sensory and optical properties of the end products to a significant degree. More particularly, high demands are placed on the color properties, such as low color number and high color stability, of the polyol esters. The structure of the starting materials, of the polyhydric alcohols and of the acids, is, in contrast, crucial for the mechanical and thermal properties of the polymer materials plasticized with the polyol esters and influences the hydrolysis and oxidation stability of lubricants.
The treatment with an adsorbent, for example activated carbon, high-surface area polysilicic acids, such as silica gels (silica xerogels), kieselguhr, high-surface area aluminium oxides and aluminium oxide hydrates, or mineral materials such as clays or carbonates, during the workup of the crude polyol ester to improve the color is a conventional process, but it requires additional filtration steps which mean a considerable level of complexity in a process performed industrially. Valuable product likewise remains adhering in the filter device and on the adsorbent, such that valuable product is lost in an additional filtration step.
Treatment with hydrogen peroxide to improve the color can also be found to be problematic since it can result in formation of organic peroxides during the treatment of the polyol esters. Traces of peroxides reduce the ester quality and the performance properties of the plasticized polymer products and of the lubricants produced on the basis of polyol esters. Peroxide traces also impair the storage performance of the polyol esters, and an increase in the peroxide number is observed during storage in spite of exclusion of oxidizing agents such as air. To reduce the peroxide number, the prior art proposes an additional treatment with a reducing agent. Although this process is capable of reducing the peroxide number, such an operation means an additional working step in which the reducing agent has to be provided and removed again after use thereof.